I agree that back should bring you back. Not sure about Gmail. First of all, it doesn't behave the way you describe on my Nexus One. It always goes to the home

I agree that back should bring you back. Not sure about Gmail. First of all, it doesn't behave the way you describe on my Nexus One. It always goes to the home screen when I switch to it, regardless of how I do it. Second, is this really the best behavior? Or is the behavior you describe the best behavior? If you get a new message notification, and you tap on it, don't you usually want to see the new message? And lastly, do you propose that applications should always implement "up" explicitly, or only when necessary? Because most of the time, they would just be duplicating functionality that already exists, thanks to the back button. But if you only show it when necessary, you're creating a situation where casual users won't really understand when and why they should use which UI element.